New frequency spectrum allocations for personal communications have created a need for portable telephones which operate both in the existing 900 MHz frequency band as well as the newly allocated 1.8 GHz frequency band. Therefore, local oscillators for these dual band portable telephones are required to operate in two widely separated frequency ranges.
Prior art dual band devices have included; completely separate local oscillators with resistive combiner networks or filter networks, local oscillators with doublers or triplers to multiply the frequency, pin diodes or other similar RF switching devices, and extremely wide band oscillators having an operable frequency range that overlaps the two bands of interest.
The disadvantage of using resistive combiner networks is that minimal isolation is achieved between the local oscillator signals and resistive combiners which causes signal losses and noise. Filters networks improve isolation somewhat, but this adds circuit complexity and cost and draws additional current.
The disadvantage of using local oscillators with doublers or triplers to multiply the frequency is that spurious signals are always present in the output. For example, for high band signals a doubler or tripler will always have some subharmonic signal that can interfere will low band signals. Similarly, for low band signals higher harmonics can interfere with high band signals. These spurious signals must be filtered out to avoid degrading receiver performance or interference with other radio services. In addition, the parts count increases greatly with doublers and triplers and the desired output frequencies must exactly match those multiples.
The disadvantage of pin diodes is that pin diodes require significant DC current to obtain a low "on" impedance, and when the pin diodes are "off" they can create high levels of harmonically related spurious signals. Moreover, tank circuits associated with the pin diodes reduce circuit Q, which reduces efficiency, and causes higher phase noise in the output circuit. Also, pin diode switching adds to circuit complexity and cost.
The disadvantage of using extremely wide band oscillators is that wideband oscillators are necessarily very sensitive to tuning control. This sensitivity makes the oscillator more susceptible to noise on the tuning control line. Correspondingly, more sensitive tuning requires tighter coupling to the tuning element (varactor) of the oscillator which causes higher losses in the associated tank circuit.
There is a need for a combiner circuit for voltage controlled oscillators that: provides good isolation between local oscillator signals. In addition, there is a need for a combiner circuit that does not require pin diodes, is not sensitive to noise on a tuning control, minimizes spurious frequency signals, has low losses and current drain, and requires simpler, and therefore less costly, circuitry.